Natural gas can be liquefied for purposes of storage and transportation, as it is occupying a smaller volume in liquid state than in gaseous state. Typically, before being liquefied, the natural gas is treated to remove contaminants (such as H2O, CO2, H2S and the like) and heavy hydrocarbon molecules, which may freeze out during the liquefaction process.
Liquefaction of natural gas is an energy consuming process. Designing and operating liquid natural gas plants in the most efficient manner is therefore a constant focus area.
U.S. Pat. No. 4,504,296 A describes a process for liquefying a natural gas stream using Dual Mixed Refrigerant (DMR) cycles. The first and second cooling stages are against a mixed multicomponent refrigerant composed of methane, ethane, propane, butane and nitrogen.
EP 1 340 951 A2 describes a process for liquefying a natural gas stream using multiple refrigerant cycles. The first and second cooling stages are against a Mixed Refrigerant (MR) while the third cooling stage can be against nitrogen. The second cooling stage is carried out in a single heat exchanger at a single pressure of mixed refrigerant.
US 2005/056051 describes a process for liquefying a natural gas stream using multiple refrigerant cycles. The first cooling stage is against a propane refrigerant, the second cooling stage is against MR and the third cooling stage can be against nitrogen. The second cooling stage is carried out in a single heat exchanger at a single pressure of MR.
DE 3521060 describes a process for liquefying a natural gas stream using three refrigerant cycles. The first and third cooling stages are against a MR or propane refrigerant, while the second cooling stage is against a MR. There is no disclosure of the use of at least two heat exchangers operating at different pressures of MR in the second cooling stage.
U.S. Pat. No. 6,253,574 B1 describes a process for liquefying a natural gas stream using a MR cascade cycle of three-MR cycles having different refrigerant compositions. The refrigerant for the first cycle is a mixture of ethylene or ethane, propane and butane. The refrigerant for the second cycle is a mixture of methane, ethylene or ethane and propane, and the third refrigerant is a mixture of nitrogen, methane and ethylene or ethane. The use of MR can have some advantages in certain situations, for example in a large Main Cryogenic Heat Exchanger (MCHE), which is efficient when cooling to below −100° C.
WO201576975 describes a method of retrofitting a full-scale Liquefied Natural Gas (LNG) plant to enhance the LNG production capacity of the LNG plant. A small-scale LNG plant having a capacity less than 2 MTPA can be integrated with a main LNG plant having a capacity of at least 4 MTPA such that end flash gas and boil off gas from the main LNG plant can be liquefied by the small-scale LNG plant as incremental LNG. According to WO201576975 the production capacity of the integrated system can be improved by increasing the temperature of the gas stream exiting the MCHE of the main LNG plant between 5° C. and 30° C. as compared with the design temperature.
WO2006009646 is related to hydrocarbon fluid processing plants, methods of designing hydrocarbon fluid processing plants, methods of operating hydrocarbon fluid processing plants, and methods of producing hydrocarbon fluids using hydrocarbon fluid processing plants. More particularly, some embodiments of the invention are related to natural gas liquefaction plants, methods of designing natural gas liquefaction plants, methods of operating natural gas liquefaction plants and methods of producing LNG using natural gas liquefaction plants. One embodiment of the invention includes a hydrocarbon fluid processing plant including a plurality of process unit module types, the plurality of process unit module types including at least a first process unit module type including one or more first process unit modules and a second process unit module type including two or more integrated second process unit modules wherein at least one of the first process unit modules and at least one of the second process unit modules are sized at their respective substantially maximum processing efficiency.
WO2001081845 A1 relates to controlling the production of a liquefied natural gas product stream comprises steps of a) measuring the temperature and the flow rate of the liquefied natural gas product stream and measuring the flow rates of the Heavy Mixed Refrigerant (HMR) and of the Light Mixed Refrigerant (LMR); b) selecting the flow rate of one of the refrigerants (HMR, LMR or the total mixed refrigerant) to have an operator manipulated set point, and generating a first output signal for adjusting the flow rate of HMR and a second output signal for adjusting the flow rate of the LMR c) adjusting the flow rates of the HMR and LMR in accordance with the first and second output signals; d) determining a dependent set point for the ratio of the flow rate of the liquefied natural gas product stream to the flow rate of one of the refrigerants such that the temperature of the liquefied natural gas product stream is maintained at an operator manipulated set point; e) maintaining the flow rate of the liquefied natural gas product stream at its dependent set point.
It is an object to improve the dynamic response of a refrigerant loop associated with a main cryogenic heat exchanger (MCHE) to relatively fast flow rate changes/variations in the feed stream (typically natural gas) of the main cryogenic heat exchanger. There is a need for such a dynamic response in situations where relatively fast flow changes/variations in the (natural) gas feed circuit occur (frequently). Relatively fast herein may mean, for instance, losing 30% vol. or more of the feed in a time frame of 1 to 2 minutes.
Such relatively fast flow rate changes/variations may for instance occur when upstream equipment fails unexpectedly. Also, such relatively fast flow rate changes/variations may occur in situations in which the feed gas stream is (partially) obtained from (the pretreatment unit of) one or more different liquefaction trains or LNG trains, in case one of those parallel liquefaction trains trip.
If not controlled well, the liquefaction train may trip and/or equipment may be damaged. Both cases typically result in significant costs and therefore losses, the first due to lost production and the second for replacement of relatively expensive equipment. For instance, liquid refrigerant (for instance MR) may drop out of the MCHE. For instance, this may lead to compressor for the refrigerant to trip. If the (resulting in train trip and restart, flaring) and large temperature gradients in the MCHE may occur, hence MCHE tube leaks. An example application is a common liquefaction unit (CLU), fed by treated gas from various in-plant (train) feed sources, considering that the feeding trains trip more often.